The Texas Horned Lizard

The Texas horned lizard is the state reptile of, you guessed it, Texas and has experienced population declines since the 1950s. There are multiple factors implicated in the decline of the species: pesticide use, collection for the pet trade, the invasive red imported fire ant, and habitat loss and degradation. Rachel Granberg is a recent masters graduate from Texas Tech University who’s research studied Texas horned lizard habitat loss and degradation in central Texas, where the species has experienced severe declines over the past 70 years. Habitat loss and degradation are caused by conversion of native prairie to agriculture or urban areas, but also by exclusion of fire from the ecosystem. Fire is a natural form of disturbance that has occurred for thousands of years in prairies, reducing the persistence of shrubs and trees and allowing a grassy understory to thrive. Historically, fire was caused by lightning strikes as well as Native Americans. With European settlement, human-caused fires were reduced and lightning-strike fires were fought. This resulted in woody plants increasing in dominance through the landscape, converting prairies to woodlands. Controlling woody plant persistence is a constant battle in central Texas and difficult as well as expensive to win without the use of fire.

Rachel’s research studied the effect of prescribed fire, fire used in a controlled setting to reduced shrub and tree presence, on the Texas horned lizard. She found that reduced shrub density and increased grass and forb diversity increased female horned lizard survival; both of these are accomplished through use of prescribe fire. As ectotherms, lizards require sunny areas to bask as well as cool areas to escape the Texas heat. Forests or woodlands with complete canopy cover do not provide a patchy mosaic of sunny and shady areas to thermoregulate. Furthermore, the harvester ant (comprising up to 90% of horned lizard diet) depends on seeds from grasses and forbs, which are more vigorous in open habitats with lower densities of woody plants. Interestingly, male horned lizard survival was influenced by primarily size, with larger males surviving longer. Because females have an additional need to regulate temperature for maturing eggs properly and also carry up to their own body weight in eggs, habitat characteristics are very important to their survival. Food (ants) must be near patchy habitat that allows them to feed and thermoregulate without expending excess energy. Prescribed fire is a cost-efficient, naturally-occurring method to restore prairies used by the Texas horned lizard (as well as other grassland species). Although there are several factors implicated in the decline of this species, prairie restoration through the use of fire can eliminate one issue that limits the success of the Texas horned lizard in central Texas.

EOC podcast episode which features an extended interview with Rachel Granberg: